Keyer circuit for electronic musical instrument

ABSTRACT

In an electronic musical instrument, a sustain keyer circuit comprises a field effect transistor, a resistor and a keyoperated switch all connected in series between a ground a power line. An input tone signal supplied to the gate of the FET is keyed to appear at the drain thereof. A capacitor is connected across the key-operated switch. Between the capacitor and the ground is connected another FET which is switched on and off by a rectangular wave having a supra-audible frequency and a variable duty factor thereby exhibiting a variable average resistance, determining a sustain time in cooperation with the capacitor.

United States Patent [191 Uchiyama 1 1 KEYER CIRCUIT FOR ELECTRONICMUSICAL INSTRUMENT Inventor:

Yasuji Uchiyama, l-lamakita, Japan Assignee: Nippon Gakki SelzoKabushiki Kaisha, Hamamatsu-shi, Shizuoka-ken, Japan Filed: Mar. 16,1972 Appl. No.: 235,350

Foreign Application Priority Data Mar. 23, 1971 Japan 46/4616632 17.5.cl..... 1 84/].26, 84/1.13, 84/D1G, 23 Int.Cl. G10h 1/02 FieldofSearch84/1.01,1.13,l.26,

" s4/mo. 23; 307/251, 303

[56] References Cited UNITED STATES PATENTS 5/1972 l-Iirano 84/].2612/1971 Hiyama 84/l.13 2/1972 Campbell 307/251 X 5/1972 Williams 307/251X Primary ExaminerRichard B. Wilkinson Assistant Examiner-Stanley J.Witkowski Att0rr1ey--John W. Malley et a1.

[ ABSTRACT In an electronic musical instrument, a sustain keyer circuitcomprises a field effect transistor, a resistor and a key-operatedswitch all connected in series between a ground a power line. An inputtone signal supplied to the gate of the FET is keyed to appear at thedrain thereof. A capacitor is connected across the keyoperated switch.Between the capacitor and the ground is connected another FET which isswitched on and off by a rectangular wave having a supra-audiblefrequency and a variable duty factor thereby exhibiting a variableaverage resistance, determining a sustain time in cooperation with thecapacitor.

7 Claims, 10 Drawing Figures led PAIENIEDJIMmn 3.746.775

SHEET 1 0F 3 FIG. I PRIOR ART PRIOR ART -Vd %RL 1 C Co 80 WHHH I- Ro IQ2 A7 a: D-VQ| D 03 R i: S G ru 0 O /77 A? I FIG. 2b

PRIOR ART 3:'i; h I i s I KE Y sw ON IKEY sw OFF TIME FIG. 20 I PRIORART I I"- I I I i A I 0.1 l FIG. 2d TIME".

PRIOR ART 5 g SHEEI 2 [IF 3 FIG 4 d R 2 3 VH RI+RZ(VD VK) 5 a. Tr D OT-+--- (KEY sw ON 1 KEY sw OFF TIME v Q v -v L RI+R( n K) FIG. 5 LBTRIANGULAR SCHMITT WAVE GEN TRIGGER KEYER CIRCUIT FOR ELECTRONIC MUSICALINSTRUMENT BACKGROUND OF THE INVENTION 1. Field of the invention Thepresent invention is concerned with a sustain keyer circuit for use inan electronic musical instruvoltages of the FETs employed.

2. Description of the priorart Among the sustain keyer circuits usingFETs which have been developed of late for use in electronic musicalinstruments, there is the known arrangement as shown in FIG. 1, whichcomprises first and second FETs Q and Q, Which are connected in seriesto each other from both DC and AC viewpointsLThe first FET Q has itssource electrode grounded, with the gate electrode being adapted toreceive 'a tone signal through an input terminal T while thedrainelectrode isconnected to the source-electrode of the second FET 0,. Thissecond FET Q, is connected, on the one hand, to a voltage source Vd viaa loadresistance element R and, on the other hand, to an output terminalT through a capacitor C, thus forming an electronic switch whichfunctions as a keyernFor thesake of keying, a series circuit consistingof a resistance element R, and akey-operated switch S, isprovided'between the voltage source Vd and thegateof thesecondFET O tothereby switch a keying voltage to be applied to the gate electrode ofvthe second-FET O by the operation of the key-operated switch 8,. Theabove-mentioned series circuit further includes a capacitor C,.connectedin parallel therewith, that is, between the. gateelectrode of .thesecond FET Q,.and the ground point from AC viewpoint (herein, it is Vd),thus forminga keying circuit having a time constant determiningfunction, so that the above-mentioned tone signal applied to the FET Q,may be derived at the output terminal T upon keying of the key-operatedswitch 8,. To the gate electrode of said second FET Q, is connected athird FET Q, at its drain electrode which serves as a sustain timecontrol circuit. The source electrode of the FET O is grounded and thegate electrode thereof is connected to a slidably movable'contact r, ofa variable resistor R, such as a potentiometer which is interposedbetween the'ground and a variable DC potential source, i.e., the

- voltage source Vd.

The above-mentioned sustain keyer circuit and the FET Q, constituting asustain time control circuit are By the operation of the variableresistor R,, the sustain time, namely, the decay time of tone outputsignals at the whole keyer circuits after the keying-off of thecorresponding number of key-operated switches S, may be adjustedsimultaneously to the desired period of time by way of the FETs 0,.

In the aforesaid arrangement, it will be noted that the drain-sourceresistance of each FET Q, will increase as, for example, the position ofthe slidable contact r, of the variable resistor R, is brought closer tothe ground side thereof, and that, therefore, the transient time ofcharging of the capacitor C,, namely, the sustain time in the electronicmusical instrument, will vary in accordance with the potential at thegate of each FET O in other words, in accordance with the position ofthe slidable contact r, of the resistor R,.

In detail, the operation of such prior art arrangement will hereunder beexplained.

The resistances of resistors R, and R, and the DC potential of the powersource Vd are set so that the FETs Q and Q, are simultaneously renderedconductive ornon-conductive, namely, on or cut-off in accordance withthe opened or closed state of the switch 8,. Along with this, wheneverthe key-operated switch S, is closed, the transient characteristic(response) in the course of the beginning of conduction of the FET Q,and the FET Q, is determined by the time constant which, in turn, isdetermined by the capacitance of the capacitor C, and the resistance ofthe resistorR, to thereby determine the build-up characteristic of thetone signal derived from the output terminal T while whenever thekey-operated switch S, is opened, the transient characteristic(response) in the course of the ending of conduction of both the FET Q,and the FET Q, is determined by the time constant which, in turn, isdetermined by both of the value of capacitance of the capacitor C, andthe value of resistance produced between the drain electrode and thesource electrode of the FET Q; to thereby obtain the desired decaycharacteristic of the thus developed tone output signal, which can betermed sustain time.

In such prior art keyer circuit arrangement, however, there arisesinconsistency in the decay times of the tone signals as the output ofthe keyer circuit following a key-cutoff due to the fact that FETsgenerally develop various gate cutoff voltages. In addition, the decayamplitude characteristic of the output tone signal does not becomeexponential and fails to provide a natural sound thereby. This will bewell understood by the following explanation with reference to FIGS. 2 athrough d. FIG. 2 a shows the gate-source voltage Vgs to drainsourcecurrent I characteristic of a P channel enhancement mode MOS type FET asthe FET Q,,where Vth represents the gate cutoff voltage thereof.Accordingly, when a gate voltage Vg as shown in FIG. 2 b is applied tothe gate of the FET Q, of the type described above, a rectangularwaveform input signal applied to the gate of the FET Q, is derived atthe output terminal T, in the form of a signal having an envelope curveas shown in FIG. 2 0. However, since the gate cutoff voltages of FETsare different from one FET to another by about :50 percent, the decaytimes will be likewise inconsistent as shown by, for example, the brokenlines X or Y in FIG. 2 e in accordance with inconsistency in gate cutoffvoltages of the FETs which are employed. The foregoing decaycharacteristic becomes substantially linear in amplitude, and willassume an upwardly curved pattern as shown by the envelope curve inlogarithmic scale (dB) in FIG. 2 d. The tone decay having such a decaycharacteristic in an electronic musical instrum ent is not desirablebecause of the lack of naturalness in the decaying pattern.

Furthermore, no consideration has been made in the prior art keyercircuit as described above with respect to the problem of signal leakagewhich develops unavoidably in case of employment of FETs through whichtone signals are keyed.

SUMMARY OF THE INVENTION 'thereat, another FET determining the sustaintime constant following the opening of a key-operated switch togetherwith a capacitor in a time constant circuit in the sustain keyer andhaving its gate to vary its equivalent resistance value in accordancewith a rectangular waveform signal applied thereto, which signal has avariable duty factor controllable by an instrument player, therebycontrolling the sustain characteristic uniformly, independently of theinconsistency in the gate cutoff voltages of the FETs employed.

A further object of the present invention is to provide sustain keyercircuits for use in an electronic musical instrument, which arecapableof reducing signal leakage occurring upon mixing a plurality ofkeyed tone signals.

A still further object of the present invention is to provide sustainkeyer circuits for use in electronic musical instruments, which arecaused to reduce signal leakage due to the capacitance between the gateelectrode and the drain electrode of an FET which is employed as thekeyer.

Other objects, features and attendant advantages of the presentinvention will become apparent from the following description when takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagramillustrating a prior art keyer circuit for use in an electronic musicalinstrument, said keyer circuit employing FETs therein.

FIG. 2 a through d are charts for explaining the operation of the priorart keyer circuit, wherein FIG. 2 a is a characteristic curve of thegate-source voltage Vgs to the drain-source current I in an enhancementmode F ET; FIG. 2 b is the waveform of a voltage which is applied to thegate G of FET 0,, shown in FIG. 1; FIG. 2 c is a waveform showing anoutput signal at the circuit of FIG. I; and FIG. 2 d is a waveformshowing an output signal at the circuit of FIG. 1 in decibels.

FIG. 3 is a circuit diagram of a sustain keyer circuit illustrating anembodiment of the present invention.

FIG. 4 is a diagram showing a waveform of an output tone signal derivedfrom the sustain keyer circuit of FIG. 3.

FIG. 5 is a circuit diagram representing a modification of a part of thecircuit shown in FIG. 3.

FIGS. 6 and 7 are circuit diagrams showing other em bodiments of thepresent invention, respectively.

It should be understood that like parts are indicated by like referencesand numerals throughout this specification.

DESCRIPTION OF THE PREFERRED EMBODIMENTS For the sake of simplicity ofexplanation, the following embodiments of the present invention will beexplained by comparing with the prior art circuit which is shown in FIG.1.

Description will hereunder be made on an embodiment of the presentinvention with reference to FIGS. 3 and 4.

Referring now to FIG. 3, there is shown a keyer circuit arrangementaccording to the present invention, which comprises: a p-typeenhancement mode FET Q with grounded source electrodes S, whose gateelectrode G is adapted to receive a tone signal through the inputterminal T, from a known tone generator such as a flip-flop circuit notshown and whose drain electrode D is connected directly to a mixing loadresistor R as well as to the output terminal T which is common to aplurality of keyer circuits dealing with respectively different tonesignals; a series circuit of a key-operated switch 8,, a resistor R anda first resistor 1 connected between the drain electrode of FET Q11 anda power source line --Vd to form a keying circuit, va second resistor Rconnected in parallel with the FET Q between the drain and sourceelectrodes thereof, a capacitor C connected between the juncture of theresistor R and the resistor R and the power line (as an A.C. ground),and a p-type enhancement mode FET Q as a switching element with itsdrain electrode connected at the connection point between the capacitorCd and the resistor R and its source electrode grounded. The FET Q22 hasits gate electrode adapted to receive a rectangular wave signal from arectangular wave generator A to be rendered alternately conductive andnon-conductive.

In operation, when the key-operated switch S is opened, the capacitor Cdis charged up by the DC power source V,, and the drain electrode of theFET Q is at the ground potential thereby, so that the FET Q ismaintained in its non-conductive state and hence in spite of a tonesignal of a rectangular waveform applied to the gate electrode of theFET Q11, no output signal may be derived at the output terminal T whilewhen the switch S is closed, the capacitor Cd will begin to dischargethrough the resistor R and the switch S, to thereby increase the drainpotential of the FET Q in the negative sense continuously, and as aresult, the FET Q is rendered conductive due to the rectangular waveformsignal applied to its gate electrode. Accordingly, with the key operatedswitch S drain potential of FET Q in its conductive state, then thesignal voltage V,- (peak to peak value of an available A.C. output)which is derived at the drain electrode of the FET Q can be expressed bythe following formula:

wherein:

0 2 enato/( 2 aina) and V represents the voltage across the capacitorCd.

FIG. 4 shows a graph of the above-mentioned signal voltage at the outputterminal T, in association with the input signal applied to the inputterminal T As will be apparent from the above-mentioned formula, theoutput signal voltage is independent of the gate cutoff voltage of theFET Q The build-up characteristic Tr of the output signal .upon keyingthe key-operated switch S is determined by a time constant, which isdetermined by the capacitance of the capacitor Cd and the resistance ofthe resistor R provided that the resistance value of the resistor R, isset as follows: R a parallel resistance composed of R R, andtheeffective resistance of the FET 0,, when viewed from its drain sidetoward the ground side. The resistor R is only for determining thebuild-up time constant, and is not necessarily significant in thisinvention.

Then, upon release of the key-operated switch S the capacitor Cd beginscharging again through the effective (average) resistance of the FET Q,and the output signal decays gradually thereby. The decayingcharacteristic of the output signal is determined by a time constant,which is determined by the capacitance of the capacitorCd and mainly theeffective drain-source resistance of the FET Q11 (also by the resistancevalue of the circuit includingthe resistor R the resistor R, and the FETQ Accordingly, the decaying characteristic of the tone signal afterkeying-off, namely the so-called sustain characteristic, can be variedby varying the effective drain-source resistance of the FET 0,, asrequired.

Referring to FIG. 5, there is shown an example circuit for controllingthe FET Q", in which arrangement is provided so that the output signalproduced in a triangular waveform having a predetermined repetitionfrequency, such as 40 kHz, which is higher than the audible frequencyand which is obtained from a triangular waveform generator G is appliedvia a DC blocking catain keys circiiit and the aforesaid FET Q22 arepropacitor C and a diode D in this order, to the input side of the knownSchmitt trigger circuit F which is comprised of elements such astransistors and resistors, and

keying circuit-of the sustain keyer circuit so that the rectangularwaveform output signal of the aforesaid predeterminedrepetition which isproduced in said Schmitt trigger circuit F is applied to the gateelectrode of the FET Q22. By the use of the aforesaid circuitarrangement, the latter is operative in such a way that, by changing theDC potential of the point 'Hbr" connection between the capacitor C andthe diode D via a variable resistor R which is interposed vided in, forexample, the console of an electronic musical instrument each in numbercorresponding to the multiplicit number of tone generators installed inaccordance with the number of the keys of the electronic musicalinstrument. It should be noted also that such elements as the triangularwaveform generator G, the variable resistor R B and the Schmitt triggercircuit F are provided one each in such a waytha'ma combination of theseelements is used in common'to said multiplicit number of FET Q tocontinuously effect simultaneosus change of the conductive time of saidmultiplicity of FET On by changing the duty factor of the rectangularwaveform signal generated at one output terminal T, of said Schmitttrigger circuit F.

Let us now assume that the circuitry is in the state such that the DClevel of an output signal form the triangular wave generator G is set ata predetermined value by means of the variable resistor R and that theduty factor of the rectangular wave produced at the output of theSchmitt trigger circuit F is set at a predetermined value to have theconducting to nonconducting time ratio of the FET Q set at apredetermined value of the order of, for example, 0.5When thekeyoperated switch S, is opened in such a state of the circuitry, thedrain electrode of the FET Q1 and the drain electrode of the FET Q areat the ground potential. So, this latter FET Q is held in its cut-offstate. Accordingly, the tone signal which is always applied to the gateelectrode of the FET Q does not appear at the drain side of the FET QDuring the foregoing operation of the circuitry, it will be underdtoodthat the capacitor C which is connected through resistor R to the drainelectrode of the FET Q is in the state of being charged up with thevoltage of the power source V,, by the repetition of the momentaryconducting which is effected between the drain electrode and the sourceelectrode of the FET O with the junction between the capacitor C, andthe FET 0,, being at a voltage potential substantially equal to theground potentialzero.

On the other hand, when the key-operated switch S, is closed, the chargeon the capacitor C which has been in the state of being charged up inthe mode of operation described above is discharged through theresistance element R, which, in turn, is connected in series with saidkey-operated switch 8,, and along with this, the potentials of both thedrain electrode of the FET Q and the drain electrode of the FET Qapproach the negative potential of the power source V,,. As a result,the FET Q will become conductive as the tone .signal which is applied tothe input of the FET Q11 is derived, as the output tone signal having abuild-up characteristic in accordance with the characteristic of thekeyer circuit, at the output terminal T, via the drain electrode of thisFET Q The tone signal thus derived may be amplified as desired to beconverted to an audible sound wave to be given out from such anappliance 'as a loud speaker not shown.

Next, when the key-operated switch S, is opened again, the capacitorC,,, which has been in the state of i being discharged in the mode ofoperation described above begins to be charged up with the voltage ofthe power source -V through the average (effective) resistance betweenthe source and the drain of the FET O which is rapidly switched to bemomentarily conducting and momentarily non-conducting, and along withthis, the drain potential of the FET Q11 as well as the drain potentialof the FET Q22 approach the ground potential. Also, when the potentialsof these two electrodes constituting a keying terminal take a value inexcess of a predetermined potential for a period of time, the FET Q willproduce no output at its drain electrode in the same manner as thatdescribed in connection with the opened state of the key-operated switch8,. Thus, it will be understood that the decaying characteristic of theoutput signal is determined by the time constant which is determined bythe combination of the capacity of the capacitor C the averageresistance resulting from the conducting to non-conducting time ratiobetween the drain and the source electrodes of the FET Q22, and a seriesresistance of resistors R and R (also incluing the FET Q The aforesaidsustain time or the decay time of an electronic musical instrument maybe controlled by varying the DC level of the triangular waveform signalapplied to the input side ofthe Schmitt trigger circuit F, by alteringthe resistance value of such an element as the variable resistor R B tothereby alter the duty factor of the rectangular waveform output signalderived at the output of said Schmitt trigger circuit F, wherebycontinuously varying both the momentary duration of cut-off time and themomentary duration of conducting time of the FET Q For example, from aviewpoint of the duty factor, when the momentary conducting period oftime is considerably smaller as compared with the momentary cut-offperiod of time, the length of charge-up time of the capacitor Cfollowing the opening of the aforesaid key-operated switch 8,, (meaningafter the key switch-off time) will accordingly become greater. Hence,the sustain time or the decay time of the tone signal which is derivedat the outputterminal T, under the aforesaid condition will beprolonged. Contrariwise, in case where the momentary conducting periodof time of the FET Q is substantially great as compared with itsmomentary cut-off period of time, the length of charge-up time of thecapacitor C, following the opening of the key-operated switch S isreduced, so that the sustain time or the decay time of the aforesaidtone signal will become shorter. In this way, the sustain time of anelectronic musical instrument can be controlled. However, in the presentinvention, the circuitry is arranged in such a way that the gatepotential of the FET Q is controlled at a very high repetition rate,such as 40 kHz so that this FET 0,, can alternately become fullyconductive and fully nonconductive. As a result, the FET Q22 can effectthe switching between its cut-off state and its conductive state at ahigh speed, regardless of the gate cut-off voltage of this FET Q- Thus,the FET O1, is always held in either the fully conducting or fullynon-conductive state, and the average (effective) internal resistancebetween the drain and the source electrodesof this FET 0,, will becaused to vary by the varying duty factor. Therefore, it is possible toconsider that the voltage to current characteristic is of a valueequivalent to that of a variable resistor having a linear voltage tocurrent characteristic. Thus, the decay characteristic of the tonesignal within the aforesaid sustain time can be obtained as one which isquite close to a complete rectilinear line as shown by the envelopestraight lines in logarithmic (dB) scale, so that the optimum auditoryexponential decay pattern can be obtained.

It is to be noted that, by the employment of only one each of therectangular waveform signal generating means such as the triangular wavegenerator G, the Schmitt trigger circuit F, the diode D and the variableresistor R,,, which are capable of varying the duty factor it ispossible 'to effect the switching of a number of FET Q assigned forcontrolling the sustain time. Thus, the circuitry of the presentinvention is of many advantages that it can be produced easily and at alow cost, which is highly useful industrially.

From the foregoing description of the present invention, it will beunderstood that a keyed tone signal may be derived at the junctionbetween the resistor R and the keyer FET Q in each keyer circuitregardless of the inconsistency in the gate cut-off voltages of the FETswhich are employed.

Practically, in an electronic musical instrument having a large numberof keyer circuits, there can arise the case that those keyed signalsderived from a plurality of keyer circuits corresponding in number tothe number of half octave tone keys are mixed, using the resistor R incommon to the group of keyer circuits. In such an arrangement, there mayoccur an undesirable signal leakage due to the possibility that thepotential of a tone output signal keyed by a keyer circuit being turnedon is given to a keyer FET Q of another keyer circuit. In order toobviate this shortcoming, a class-A buffer amplifier including an FET Qis provided, as shown in FIG. 6, in each keyer circuit of FIG. 3. To thegate electrode of the FET Q33 is connected the drain electrode of theFET Q11, and to the drain electrode of the FET Q33 is connected thecommon load resistor R as mentioned above. Whereby, the intended tonesignal mixing can be performed without a separate damper of signalleakage, since a gate current in FETs is in the order of 10 ampere inmagnitude. A resistor R connected to the source electrode of the FET Q33serves to reduce the inter-modulation distortion in the resulting tonesignal which may take place due to the changes in the resistance of theFET Q when looking at from the drain side thereof, which changes arecaused by the variations in the drain potential of other keyer circuits.

FIG.-7 shows a further embodiment of the present invention which isintended to reduce as much as possible the signal leakage occurring dueto the gate-drain capacitance of the keyer FET Q as a rectangularwaveform signal is applied to the input of this keyer FET. The signalleakage is considered to be caused because the applied rectangularwaveform contains very high harmonics, in other words, because suchrectangular wave contains harmonics over the range up to about 1 MHzclearly.

For the purpose of eliminating the signal leakage, the circuit of FIG. 6is further provided with another FET 0 between the keyer FET Q and theinput terminal T in each keyer circuit to become of a circuit as shownin FIG. 7. To the gate electrode of the FET Q11 is connected the drainelectrode of the FET Q44 nd a capacitor at'its one end and grounded atthe other end. The input terminal T is connected to the source electrodeof the FET Q whose gate electrode is connected to the power source V,,.The FET Q which is thus arranged is employed as a resistance elementcapable of exhibiting a high resistance value, for example, M 0.. Thecapacitor C is set to have the capacitance of 1 pF, for example. Thus,there is formed a kind of low pass filter which comprises thedrain-source resistance of the FET Q and the capacitance of thecapacitor C to thereby eliminate higher harmonics of the rectangularwaveform input signal beyond the order of, for example, 16 kHz,resulting in an extreme reduction of signal leakage due to thegate-drain capacitance of the keyer FE Q Furthermore, even if a mansfinger contacts the input terminal T there arises no danger of causingthe breakdown of the FET 0,, caused by an electro statical shock, sincea high resistance of the FET Q is interposed between the input terminalT, and the FET Qu.

As willbe clear from the foregoing description, the novel keyer circuitarrangement of the present invention not only provides desired keyercharacteristics by overcoming the problem of inconsistency in the gatecut-off voltages of the FETs employed, but also it can prevent a signalleakage which can take place upon the mixing of keyed tone signals ofdifferent keyer circuits and can reduce any signal leakage source line;from the gate-drain capacitance of each keyer FET. It is to be notedthat the above-mentioned circuit arrangement is very much Suitable forand convenient to circuit integration (IC production).

I claim:

1. In an electronic musical instrument, a sustain keyer circuitcomprising:

means establishing a circuit ground; v

a power source;

an input terminal for receiving a tone signal to be keyed;

an output terminal for deriving a keyed output;

a first field effect transistor having a source electrode connected tosaid ground means, a gate electrode connected to said input terminal anda drain electrode connected to said output terminal;

a resistor and a key-operated switch connected in series between saiddrain electrode and said power source line;

a capacitor connected across said switch;

a switching element having an inherent internal resistance and connectedbetween said capacitor and said ground means; and

a rectangular wave generator for generating a rectangular wave signalhaving a frequency above audible range and a duty factor variablycontrolled by the instrument player, said generator being connected tosaid switching element for rendering said element alternately conductiveand nonconductive in response to said rectangular wave therebyexhibiting an average resistance, which resistance and gate and thecapacitance of said capacitor cooperatively determining a decaying timeconstant of the circuit.

2. The keyer circuit according to claim 1, further comprising:

a second FET having its gate connected to the drain of said first FETand its drain connected to the output terminal and a load resistor forenabling the mixing of keyed tone signals coming from other keyercircuits respectively of the same construction as said keyer circuit,the drains of the respective second FETs being connected to said outputterminal and the load resistor.

3. The keyer circuit according to claim 2, in which a third FET isprovided between said input terminal and the gate of said first FET forexhibiting a high resistance and a capacitor is connected between thegate and the source of said first transistor, thereby reducing signalleakage of the keyed tone signals due to the gatedrain capacitance ofsaid first FET.

4. The keyer circuit according to claim 1,'in which:

said switching element is an FET whose drain is connected to saidcapacitor, and whose gate is connected to said rectangular wavegenerator.

5. The keyer circuit according to claim 4, in which:

said rectangular wave generator comprises a triangular wave generatingcircuit, a Schmitt trigger circuit adapted to receive the output signalsof a repetition freuquency coming from said triangular wave generatingcircuit and also to generate a rectangular wave, and means for varying aDC input potential applied to said Schmitt trigger circuit.

6. The keyer circuit according to claim 5, in which:

said means for varying the DC input potential comprises a variableresistor connected to said Schmitt trigger circuit for varying the DCinput potential of the Schmitt trigger circuit.

7. A keyer circuit according to claim 1, in which a further resistor isprovided between the drain electrode and the source electrode of saidfirst field effect transistOl'.

1. In an electronic musical instrument, a sustain keyer circuitcomprising: means establishing a circuit ground; a power source; aninput terminal for receiving a tone signal to be keyed; an outputterminal for deriving a keyed output; a first field effect transistorhaving a source electrode connected to said ground means, a gateelectrode connected to said input terminal and a drain electrodeconnected to said output terminal; a resistor and a key-operated switchconnected in series between said drain electrode and said power sourceline; a capacitor connected across said switch; a switching elementhaving an inherent internal resistance and connected between saidcapacitor and said ground means; and a rectangular wave generator forgenerating a rectangular wave signal having a frequency above audiblerange and a duty factor variably controlled by the instrument player,said generator being connected to said switching element for renderingsaid element alternately conductive and nonconductive in response tosaid rectangular wave thereby exhibiting an average resistance, whichresistance and gate and the capacitance of said capacitor cooperativelydetermining a decaying time constant of the circuit.
 2. The keyercircuit according to claim 1, further comprising: a second FET havingits gate connected to the drain of said first FET and its drainconnected to the output terminal and a load resistor for enabling themixing of keyed tone signals coming from other keyer circuitsrespectively of the same construction as said keyer circuit, the drainsof the respective second FET''s being connected to said output terminaland the load resistor.
 3. The keyer circuit according to claim 2, inwhich a third FET is provided between said input terminal and the gateof said first FET for exhibiting a high resistance and a capacitor isconnected between the gate and the source of said first transistor,thereby reducing signal leakage of the keyed tone signals due to thegate-drain capacitance of said first FET.
 4. The keyer circuit accordingto claim 1, in which: said switching element is an FET whose drain isconnected to said capacitor, and whose gate is connected to saidrectangular wave generator.
 5. The keyer circuit according to claim 4,in which: said rectangular wave generator comprises a triangular wavegenerating circuit, a Schmitt trigger circuit adapted to receive theoutput signals of a repetition freuquency coming from said triangularwave generating circuit and also to generate a rectangular wave, andmeans for varying a DC input potential applied to said Schmitt triggercircuit.
 6. The keyer circuit according to claim 5, in which: said meansfor varying the DC input potential comprises a variable resistorconnected to said Schmitt trigger circuit for varying the DC inputpotential of the Schmitt trigger circuit.
 7. A keyer circuit acCordingto claim 1, in which a further resistor is provided between the drainelectrode and the source electrode of said first field effecttransistor.